A team of scientists at the Oregon National Primate Research Center and the Oregon Health & Science University are reporting a remarkable advance in the treatment of inherited genetic disease in the journal Nature.

They show it is possible to repair a tiny part of a human egg cell that, when broken, causes a host of awful inherited genetic diseases. Those diseases cause disability and the death for many children and adults. What is equally remarkable is that the treatment they report is illegal in Britain, Germany, Costa Rica, Norway and Sweden and would be illegal to provide using federal dollars in the United States.

What did the Oregon scientists do? And why is it so ethically controversial?

Mitochondria are the batteries of human cells. They convert oxygen and nutrients into a chemical that is the source of the energy that allows chromosomes to move and recombine and, once a sperm arrives, a fertilized embryo to grow. Every cell in your body has mitochondria inherited from your mother’s egg. When these little cellular engines have a genetic problem, it can make for terrible diseases in any child that inherits them.

About 4,000 U.S. children are born each year with mitochondrial diseases. They may become blind, paralyzed or suffer severe cognitive impairment.

The Oregon team showed that they could fix the problem of damaged mitochondria in an egg cell by transplanting the DNA in the nucleus of eggs with diseased mitochondria into eggs with healthy mitochondria that had the nucleus removed. They did this both in monkeys and humans. A child born as a result of this genetic transplant—referred to as a spindle transfer-- would be the genetic offspring of two mothers.

The child would have healthy mitochondrial DNA from the egg of a donor mom. He or she would also have DNA from the mom with the mitochondria problem.

This is amazing genetic engineering. Gene transfer in human eggs has and will provoke a lot of controversy.

When the Oregon team did their studies they proved the transplanted genes would work in a human egg by making them into viable embryos. Those embryos were studied in various ways to prove they were normal -- and then destroyed. There is no other way to prove that transplanting genes between eggs could someday cure children of terrible diseases without the kind of experiment the Oregon group did.

While monkeys have been used in the past, this is the first paper reporting success in the genetic engineering of human eggs.

But to go forward, more such experiments will need to be done to ensure the safety of the technique. Currently there is a ban in the U.S. on doing this kind of research with federal funds. Federal law forbids using federal taxpayer dollars to pay for any research involving the destruction of a human embryo.

This experiment also crosses a bright ethical line. Changing genes in the lungs of people with cystic fibrosis or in the eyes of people with retinitis pigmentosa or macular degeneration may fix the broken body part, but the change is not passed on to future generations.

When you change genes in an egg, even in the mitochondria of an egg, you make a change that is inherited by every single offspring of any child created from that egg. That is called germline engineering -- meaning changing inherited genetic material.

And germline engineering of mitochondria crosses the line from using genetic engineering to fix our body parts into directly engineering the traits of our children. It is a road that could lead, in the distant future, toward eugenics.

So should we celebrate or condemn this first step into reproductive cell or germline genetic engineering?

I think the price of experimenting on embryos to find a solution to disease, while high, is morally acceptable. Creating embryos in the future by means of a mitochondrial transplant that will not be used to make babies on a limited basis seems to appropriately value children and adults over possible children and adults.

And while I, too, worry about where genetically engineering eggs might lead, I think doing so to find cures is ethically noble. Those nations that say no to any form of germline engineering, including the U.S., should revisit those policies to permit research that is clearly intended as therapy.

The brave new world has now appeared in print. We need to be brave enough to avail ourselves of the good it can bring.

Arthur Caplan is thehead of the Division of Medical Ethics at NYU Langone Medical Center.